1. Field of the Invention
The present invention relates generally to an optical cross-connect for setting and switching over a route of photo signals by use of an optical switch, and a method of switching over the optical path by use of this optical cross-connect. The present invention relates more particularly to an optical cross-connect and a using method thereof, which are capable of confirming a connectivity of the route concerned when switching over the route of the photo signals. The present invention also relates to an optical ADM (Add/Drop Multiplexer) using the optical cross-connect, and to an optical cross-connect network system.
2. Description of the Related Art
In recent years, an optical ADM (Add/Drop Multiplexer) using an optical switch and an optical cross-connect network system are combined with a wavelength division multiplexing (WDM) technology, and are thereby capable of processing a large capacity of signals. Besides, the large capacity of signals can be switched over by the optical switch, and hence setting of a signal route (path) can be facilitated. This being the case, a variety of studies and developments of the optical cross-connect network system have been made.
A technical emphasis of the optical cross-connect network system has been so far placed on a point of how much efficiently a node-to-node signal route can be set and functions such as a protection can be actualized. On the occasion of structuring the optical cross-connect network system described above, however, an operation in the case of switching over the route of the photo signals, especially the operation in case a system failure and mis-setting happen, are not necessarily objects to be examined.
A literature cited showing the construction of this type of optical cross-connect described above may be exemplified such as, e.g., Chungpeng Fan, xe2x80x9cExamining an integrated solution to optical transport networking.xe2x80x9d, Wavelength Division Multiplexing: (The first ever European meeting place for WDM Systems, Network, Marketing and Engineering Professionals), November 1997, London; reference pages: pp. 18-23, Satoru Okamoto et al., xe2x80x9cOptical path cross-connect node architectures for photonic transport network.xe2x80x9d, Journal of Lightwave Technology, Vol. 14, No. 6, June 1996, pp. 1410-1422, FIGS. 4, 12.
In the conventional optical cross-connect, however, if the mis-setting is done in the optical switch, the mis-setting can be corrected for the time being. However, a given period of time is needed till the mis-setting is corrected since there was received an alarm that a desired service signal is cut off, resulting in such a problem that the service signal 1 is temporarily cut off. In the case of switching over the optical path for the photo signals, it is of much importance to confirm beforehand a connectivity of the optical path. Nevertheless, there has been no example in which that was recognized as a subject and specifically examined.
It is a first object of the present invention to provide an optical cross-connect capable of enhancing a reliability on an optical transmission system using an optical switch by making it feasible to confirm a connectivity before switching over an optical path.
To accomplish the above object, according to a first aspect of the present invention, an optical cross-connect comprises at least two photo signal input terminals for respectively inputting photo signals, at least two photo signal output terminals for respectively outputting the photo signals, and an optical switch for switching over an optical path between the photo signal input terminal and the photo signal output terminal. The optical switch incorporates a bridge connecting function of, if the optical path is switched over to a switchover target photo signal output terminal to which an optical path is newly connected from a pre-switchover photo signal output terminal through which the photo signal input terminal and the optical path are connected before the switchover, temporarily connecting the optical path to both of the pre-switchover photo signal output terminal and the switchover target photo signal output terminal.
The optical cross-connect of the present invention further comprises a second monitor circuit, disposed between the optical switch and the photo signal output unit, for monitoring a state of the output photo signal outputted from the optical switch. The optical cross-connect still further comprises a first monitor circuit, disposed between the optical switch and the photo signal input terminal, for monitoring a state of the input photo signal inputted to the optical switch.
A contrivance of the optical cross-connect according to the present invention is, in the case of switching over the optical path between the I/O terminals of the photo signals, not that the optical path is not switched over at one time but that there is performed a bridge connection of temporarily connecting the optical path to both of the pre-switchover photo signal output terminal and the switchover target photo signal output terminal. The monitor circuits for monitoring a state of the photo signals are disposed anterior and posterior to the input terminal and the output terminal of the optical switch, and are capable of monitoring the connectivity of the optical switch by comparing the states of the photo signals before and after the switching over the optical path with each other, especially the state of the photo signal outputted to the output terminal of the switch over target output terminal with the state of the photo signal before inputting to the optical switch.
In particular, with the connectivity monitor circuit being provided, it is feasible to monitor the connectivity of the optical switch from the connectivity information contained in the output photo signal and the input photo signal. The connectivity monitor circuit, if the connectivity information satisfies a predetermined fiducial quality of signal, outputs a control signal to a control circuit so that the optical switch executes a complete switchover from the pre-switchover photo signal output terminal to the switchover target photo signal output terminal.
Herein, the optical cross-connect of the present invention further comprises a photo signal cut-off unit, disposed between the photo signal input terminal and the optical switch, for cutting off the photo signal inputted to the optical switch from the photo signal input terminal. With this arrangement, other signals are inhibited from being inputted to the same optical path during the bridge connection. The control circuit controls the photo signal cut-off unit to cut off the photo signal inputted to the photo signal cut-off unit corresponding to the photo signal input terminal connected to the switchover target photo signal output terminal before switching over the optical path. Note that the connectivity information may be either an optical level of each of the input photo signal and the output photo signal or header information added to the input photo signal and to the output photo signal.
A wave-guide type optical switch can be applied as the optical switch incorporating the bridge connecting function used for the optical cross-connect of the present invention. A wave-guide type optical switch with a substrate composed of lithium niobate may be exemplified as the wave-guide type optical switch.
The optical cross-connect of the present invention may take such a configuration that the first monitor circuit includes a first optical splitter for splitting a part of the input photo signal and outputting the split input photo signal, and a light receiving element for monitoring the split input photo signal. The optical cross-connect may also take such a configuration that the first monitor circuit includes an optical level monitor circuit for monitoring an optical level of the input photo signal, a photoelectric converter for converting the input photo signal into an electric signal, and an electro-optic converter for converting the electric signal into a photo signal. Still another configuration which can be conceived is that the first monitor circuit includes a photoelectric converter for converting the input photo signal into an electric signal, an electric signal monitor circuit for monitoring the electric signal, and an electro-optic converter for converting the electric signal into a photo signal. Further, in the optical cross-connect, the first monitor circuit includes a photoelectric converter for converting the input photo signal into an electric signal, a header terminating circuit for terminating a header added to the photo signal, and an electro-optic converter for converting the electric signal into a photo signal.
According to a second aspect of the present invention, an optical cross-connect comprises a transmission-path-side input terminal for inputting a transmission path input photo signal transmitted from an optical transmission path, a transmitter-side input terminal for inputting a transmission photo signal transmitted from an optical transmitter, a transmission-path-side output terminal for outputting a transmission path output photo signal transmitted to the optical transmission path, a receiver-side output terminal for outputting a receiving photo signal transmitted to an optical receiver, and an optical switch for switching over an optical path between a photo signal input terminal including the transmission-path-side input terminal and the transmitter-side input terminal, and a photo signal output terminal including the transmission-path-side output terminal and the transmitter-side output terminal.
In the optical cross-connect having this construction according to the present invention, the optical switch has a bridge connecting function of, when the optical path is switched over to a switchover target photo signal output terminal to which the optical path is newly connected from a pre-switchover photo signal output terminal through which the optical path is connected to the photo signal input terminal before the switchover, temporarily connecting the optical path to both of the pre-switchover photo signal output terminal and the switchover target photo signal output terminal.
The optical cross-connect according to the second aspect of the present invention likewise further comprises a photo signal cut-off unit for cutting off an input of the transmission photo signal to the optical switch during a period for which the bridge connecting function of the optical switch works to temporarily connect the optical path to both of the pre-switchover photo signal output terminal and the switchover target photo signal output terminal. The connectivity information, the monitor circuits and the optical switch may take the same structures as those described above.
According to a third aspect of the present invention, an optical ADM (Add/Drop Multiplexer) can be constructed of a plurality of optical cross-connects of the present invention on the premise that the transmission path input signal, the transmission photo signal, the receiving photo signal and the transmission path output signal are inputted to one optical cross-connect and classified as photo signals belonging to the same wavelength band, and that the transmission path photo signals inputted to each optical cross-connect have wavelengths different from each other. Under this premise, the optical cross-connect may comprise an optical demultiplexer for demultiplexing transmission path input photo signals which are wavelength-multiplexed and outputting the demultiplexed photo signals to each of the optical cross-connects, and an optical coupler for coupling the transmission path output signals and outputting the coupled photo signals, thus constructing the optical ADM.
Furthermore, an optical transmission device can be also constructed of the optical ADM, described above. The optical ADM comprises a transmitting unit disposed at a transmitting-side terminal of the transmission path, and a receiving unit disposed at a receiving-side terminal of the transmission path. The transmitting unit includes an optical transmission terminal node for transmitting the photo signals belonging to the respective wavelength bands, and a transmitting-side optical coupler for coupling the photo signals and transmitting wavelength-multiplexed photo signals to the transmission path. On the other hand, the receiving unit includes an optical demultiplexer for demultiplexing the wavelength-multiplexed photo signals into the photo signals belonging to the respective wavelength bands, and an optical receiving terminal node for receiving the respective photo signals. In addition to the transmitting unit and the receiving unit, at least one optical ADM of the present invention is disposed on the transmission path, whereby the optical transmission device can be structured by use if the optical ADM.
According to a fourth aspect of the present invention, an optical ADM comprises, as a basic construction, a plurality of transmission-path-side input terminals for inputting transmission path input photo signals transmitted respectively from a plurality of optical transmission paths, a transmitter-side input terminal for inputting a transmission photo signal transmitted from an optical transmitter, transmission-path-side output terminals for outputting the transmission path output photo signals respectively outputted to the plurality of optical transmission paths, a receiver-side output terminal for outputting the receiving photo signal transmitted to an optical receiver, and an optical switch for switching over an optical path between a photo signal input terminal including the transmission-path-side input terminal and the transmitter-side input terminal, and a photo signal output terminal including the transmission-path-side output terminal and the receiver-side output terminal.
In such a construction, the optical switch has a bridge connecting function of, when the optical path is switched over to a switchover target photo signal output terminal to which the optical path is newly connected from a pre-switchover photo signal output terminal through which the optical path is connected to the photo signal input terminal before the switchover, temporarily connecting the optical path to both of the pre-switchover photo signal output terminal and the switchover target photo signal output terminal.
The optical transmission device using the optical ADM involves the use of the photo signals having the single wavelength but can be applied to the wavelength multiplexing transmission using the photo signals belonging to the wavelength bands different from each other. In the above-described construction, the transmission path input photo signals are defined as the wavelength-multiplexed photo signals in which the photo signals belonging to the wavelength bands different from each other are wavelength-multiplexed. The optical cross-connect incorporating the optical ADM function includes the optical transmitters the optical receivers, the transmission path input terminals, the transmission path output terminals, the transmitter-side input terminals and receiver-side output terminals, of which the numbers each correspond to the number of the photo signals belonging to the wavelength bands. The optical cross-connect further includes an optical demultiplexer, disposed between each of the transmission paths and the photo signal input terminal, for demultiplexing the wavelength-multiplexed photo signals into the photo signals having the respective wavelength bands, and an optical coupler, disposed between the photo signal output terminal and the transmission path, for wavelength-multiplexing the photo signals and outputting the wavelength-multiplexed photo signals to the transmission path.
Under this construction, the optical switch has a bridge connecting function of, when the optical path is switched over to a switchover target photo signal output terminal to which the optical path is newly connected from a pre-switchover photo signal output terminal through which the optical path is connected to the photo signal input terminal before the switchover between the photo signal input terminal and the photo signal output terminal of the photo signals belonging to the same wavelength band, temporarily connecting the optical path to both of the pre-switchover photo signal output terminal and the switchover target photo signal output terminal.
In the optical ADM of the present invention, in the same way as what has already been described, the monitor circuits are disposed between the optical switch and the photo signal output terminal and between the photo signal input terminal and the optical switch, and monitor the states of the photo signals before and after switching over the optical path. With this configuration, the connectivity monitor circuit is capable of monitoring the connectivity of the optical switch from the connectivity information contained in the output photo signal and the input photo signal. Similarly, the photo signal cut-off unit is disposed between the photo signal input terminal and the optical switch, whereby the photo signal inputted to the optical switch from the photo signal input terminal can be cut off.
The plurality of optical ADMs are arranged and connected as a network through the transmission paths, whereby an optical cross connect network system can be constructed.